Method and apparatus for production of rubber dispersible pellets

ABSTRACT

A pelletized form of silica which is sufficiently dust free and highly dispersible in rubber formulations. The pelletized silica is produced by a process which involves pelletizing a mixture of silica and water in an inclined mixer and drying the pelletized product in a fluidized bed dryer.

TECHNICAL FIELD

The present invention relates to an apparatus and process for producingpelletized pigments, fillers, reinforcement agents and similar rubberadditives, and more particularly relates to methods and apparatus forproducing dust-free pelletized silica which is highly dispersible inrubber compositions.

BACKGROUND ART

Rubber additives such as pigments, fillers, reinforcement agents,processing aids and the like are generally processed to be highlydispersible in rubber compositions. In the past, such additives weremade highly dispersible by being formed into fine powders. These finepowders, although meeting the requirements of being easily dispersible,pose significant problems in handling and may potentially cause healthproblems for workers who are exposed to the dust products of thesematerials.

The use of silica additives has become increasingly important to therubber industry. Silica, which may be in the form of silica pigments, iscommonly used in the rubber industry to provide reinforcing andstiffening properties to various types of rubber compositions.Additionally, incorporating silica into rubber compositions has beenfound to be advantageous in that such compositions do not deteriorateunder heating as rapidly as do most other rubber additives.

Silica in a free flowing powder form will disperse readily into rubbercompounds. However, when utilizing conventional silica products, theirdustiness becomes a problem due to the handling and compounding ofrubber formulations. Concerns by manufacturers who utilize any smallparticle size material, such as silica pigment, have led to thedevelopment of more dust-free forms of such products.

The most common method used to reduce the dustiness of compounds isparticle agglomeration or pelletization. A pelletized product is easilyhandled in a production environment and creates little dust.

A pelletized version of precipitated silica will minimize dust problemsand facilitate handling and therefore would be highly desirable torubber manufacturers. However, there remains an important requirementthat such a pelletized form of the silica must break down and redispersein the powder form. Attempts to improve silica products for use inrubber manufacturing include chemically treating silica in order toaffect good dispersibility. Heretofore an acceptable dust-free, highlydispersible pelletized form of rubber additive, particularly silica, hasnot been developed for the rubber industry.

The present invention addresses the need of a dust-free, highlydispersible pelletized form of silica and is directed to a uniqueprocess and apparatus by which such silica may be produced.

DISCLOSURE OF THE INVENTION

It is accordingly one object of the present invention to provide for aform of easily dispersible, dust-free rubber additives for use in rubbercompounding operations.

A further object of the present invention is to provide a pelletizedsilica product.

Another object of the present invention is to provide a pelletized formof silica which is highly dust-free and easily dispersible in rubberformulations.

A still further object of the present invention is to provide methodsand apparatus for producing dust free, highly dispersible rubberadditives.

An even further object of the present invention is to provide a methodfor producing a pelletized silica product.

According to the present invention there is provided a method forproducing dust free pelletized silica products which incorporates anovel mixing operation for formation of the silica pellets.Additionally, the present invention provides for a specific dryingoperation used in conjunction with the mixing operation whereby thepelletized silica is formed having the desired properties. The inventionalso provides an apparatus for forming the pelletized silica.

With these and other objects in view, the present invention will bebetter understood from the description and the claims which followhereafter taken with reference to the annexed drawings.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described with reference to theannexed drawings, which are given by way of non-limiting examples only,in which:

FIG. 1 is a diagram illustrating the method and apparatus used in oneembodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a pulley arrangement utilizedin conjunction with one embodiment of the present invention.

FIG. 3 is a graphical illustration of the relationship between theparticle size of the pelletized silica and the dispersion rating of theproduct.

FIG. 4 is a graphical illustration of the relationship between thedensity of the pelletized silica and the dispersion rating of theproduct.

FIG. 5 is a graphical illustration of the relationship between thenephelometric turbidity of a pelletized silica and the product's dustpercentage.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is directed to a method and apparatus forproducing pelletized rubber additives, particularly pelletized silica.The pelletized silica produced by the present invention is sufficientlydust-free to be easily dispersible in rubber compound formulations.

In developing the pelletized silica of the present invention variousmixing, pelletizing and drying processes and apparatus wereexperimentally evaluated. Accordingly, it was discovered that only thedisclosed, highly selective process and apparatus of the presentinvention was able to produce pelletized silica which was sufficientlydust-free and highly dispersible to be used in rubber compoundingmixtures.

By utilizing a combination of an inclined mixer and a fluidized beddryer and properly controlling the process variables and parameters asdiscussed in detail below, a pelletized silica product having less than0.8% dust contents and being highly dispersible in rubber compoundingmixtures was produced. This pelletized silica product has a densitybetween about 0.25 and about 0.40 g/cc, and average particle sizebetween about 40 and about 80 mesh, and a moisture content of less thanabout 10% by weight.

The pelletized produced is product from a feed of amorphous silica inany form such as slurry, wet cake, dry particles, etc. To produce thepelletized product amorphous silica is combined with a solvent, such aswater to form a slurry and pelletized in an inclined mixer. As discussedin detail below, the above properties of the final pelletized silicaproduct are controlled by controlling a particular combination ofprocess variables including the percentage of solvent in the mixer, themixing speed and mixing time.

The pelletized silica produced in the inclined mixer is discharged fromthe mixer and conveyed to a vibrating fluidized bed dryer in which it isdried so as to have a moisture content below 10% by weight.

FIG. 1 is a diagram which illustrates a preferred embodiment of themethod and apparatus used for producing pelletized silica according tothe present invention. As illustrated in FIG. 1, amorphous silica is fedfrom a suitable supply means 1 to mixer 2. For illustrative purposesFIG. 1 depicts an embodiment wherein dry amorphous silica is fed from ahopper having a vibrating discharge means 3. However, for purposes ofthe present invention any form of amorphous silica may be utilized as afeed material including a slurry, spray dried form, milled particles,wet cakes, etc. The only limiting parameter as regards the form of theamorphous silica feed, as will be discussed below, is the amount ofsolvent which is combined together with the amorphous silica feed inmixer 2.

Whereas FIG. 1 illustrates a supply and feed means suitable for use witha dry form of amorphous silica, when using other forms of amorphoussilica such as a slurry, wet cake, etc., conventional means andequipment required for supplying such feed could be easily incorporated.

In addition to receiving the amorphous silica feed, mixer 2 alsoreceives a solvent feed from solvent source 4 which, as illustrated, maybe a conventional supply tank connected by a suitable flow meteringmeans to mixer 2. In the pelletization of amorphous silica as furtherdiscussed below, water was supplied and used as the solvent.

As illustrated in FIG. 1, mixer 2 is an inclined, high intensity mixer.A number of types of mixers were tested during the course of the presentinvention and found to be unacceptable in that a highly dispersibleproduct could not be produced. In varying all possible operationalparameters such as percent solvent, mixing speed, residence time, etc.it was found that the proper product characteristics, i.e.dispersability and dustiness, could only be achieved utilizing aninclined, high intensity mixer as illustrated in FIG. 1. A particularlysuitable inclined mixer for the present invention is an inclined mixerin which both the mixer tank and stirrer are rotated in the same oropposite directions. In this regard, an Eirich mixer, model R-18, asdescribed in AUFBEREITUNGS-TECHNIK No. 12/75, incorporated herein byreference, was found to be particularly suitable for purposes of thepresent invention.

The inclined mixer was found to be versatile enough to produce pelletsof different sizes and densities consistently with minimal operatorinvolvement. Three main independent variables which were found effectiveto produce pellets of different densities and sizes included the percentof solvent added to the total batch weight, the rotor speed of the mixerand the peak current requirements of the mixer motor during a batchoperation. Suitable solvents include liquids capable of formingsuspensions with the material to be pelletized. In a preferredembodiment water was utilized as the solvent for pelletizing silica.

It was discovered that the rotational speed of the mixer was directlyrelated to the density of the pellets produced. Control of the speed ofthe mixer was achieved during the present invention by incorporating apulley arrangement which is schematically illustrated in FIG. 2. Asillustrated in FIG. 2, the pulley configuration included four pairs ofpulleys that were positioned between the rotor of motor 5 and the rotorof stirrer 6. Aligned pairs of pulleys between the stirrer rotor, n₂ andthe rotor n₁ had the following diameters respectively 95 in: 75 in; 110in: 60 in; 60 in: 110 in and; 75 in: 95 in. These pairs of pulleydiameters allowed for reduction ratios of 0.789, 0.545, 1.833 and 1.266.The combinations of pulley pairs are referenced by 1B, 1A, 2B and 2A asillustrated in FIG. 2. This pulley arrangement allowed for multipleconfigurations of pulleys between the two rotors of motor 5 and stirrer6 as illustrated. The faster the tip speed of the stirrer rotor or rotorrpm, which is related to a particular pulley configuration, the higherthe density of the pellet products. Table 1 below lists the rpm and tipspeeds for various pulley configurations utilizing a star-type stirringtip.

                                      TABLE 1                                     __________________________________________________________________________    RPM AND TIP SPEEDS FOR ROTORS 1 AND 2 FOR                                     DIFFERENT PULLEY CONFIGURATIONS                                               Pulley Pulley dia. (in.)                                                                           Motor Speed                                                                          Stirrer Rotor                                                                        Tip Speed                                  Arrangement                                                                          Motor                                                                             Stirrer                                                                           Reduction                                                                           RPM    RPM    (m/sec)                                    __________________________________________________________________________    1A     60  110 0.545  880    480   3.9                                        1A     60  110 0.545 1760    960   7.8                                        1B     75  95  0.789  880    695   7.8                                        1B     75  95  0.789 1760   1389   11.3                                       2A     95  75  1.266  880   1115   9.0                                        2A     95  75  1.266 1760   2230   18.1                                       2B     110 60  1.833  880   1615   13.1                                       2B     110 60  1.833 1760   3225   26.2                                       __________________________________________________________________________

Although for purposes of the present invention rotor speed wascontrolled by a combination of multiple rotors having different pulleyconfigurations, suitable control of rotor or tip speed could also beachieved by utilizing a conventional variable speed motor connected tothe stirrer 6.

It was found during the course of the present invention that the percentof solvent utilized in a particular batch was inversely related to thedensity of the pellets produced. In this regard it was discovered thatlarger percentages of solvent produced pellets having lower densities.Additionally, it was discovered that the percentage of solvent of aparticular batch mixture mixed at a particular mixing speed, i.e. aparticular pulley configuration, was also inversely related to the timeof the batch. Thus, higher solvent percentages produced acceptableproducts in shorter time periods. Table 2 below lists the variableswhich were found to effect the density of the pellets for a particularbatch. As shown in Table 2, a density greater than 0.37 g/cc could havebeen achieved by using the 2B pulley arrangement, by using a lowerpercentage of water, and by using a longer batch time. However, forpurposes of the present invention it was decided that a maximum batchtime should be 30 minutes. As noted in Table 1, water was utilized asthe preferred solvent in the present invention where producingpelletized silica. However, other liquid solvents capable of formingsuspensions with the material to be pelletized could also be utilized.

                  TABLE 2                                                         ______________________________________                                        Mixing Variables and Resultant Densities of Pelletized Silica                             Pulley Arrangement                                                            2B   2B         1A     1A                                         ______________________________________                                        % Water       66     56         69   64                                       Time (min)    2      30         2    30                                       Density       .28    .37        .25  .29                                      (pour, g/cc)                                                                  Current (amps)                                                                              2.4    2.4        1.8  1.8                                      ______________________________________                                    

In utilizing the combination of pulley configurations to control therotational speed of the rotor it was discovered that the size of thepellets was functionally related to the amperage drawn by the rotormotor 5 during a given batch run. In this regard the size of the pelletswas directly related to the amount of current drawn such that largersized pellets were produced at higher amperages. In order to effectivelymonitor the relationship between the amperage drawn by the motor and thesize of the particles during a given batch operation an ammeterconnected to the motor 5 was continuously monitored until the desiredamperage was reached for a particular particle size. When the desiredamperage was reached the batch operation was stopped and the particlesize of the pellets was measured.

Table 3 below illustrates the effect of amperage on particle size. Asshown in Table 3, at an ammeter reading of 2.2 a majority of the pelletswere formed on a 40 mesh screen (61.9%). When the motor was allowed toreach an ammeter reading of 2.3 amps, the majority of the pellets werefound on the 20 mesh screen (50.7%). If, however, the batch was stoppedafter the ammeter reading was 2.1 amps, then a large percentage of thepellets were found on the smaller 80 mesh screen (41.1%).

                  TABLE 3                                                         ______________________________________                                        Amperage vs. Mesh Size of Pelletizing Silica                                             Amperate                                                           Mesh Size (%)                                                                              2.1          2.2    2.3                                          ______________________________________                                         6           .3           .6     5.2                                          10           1.1          1.7    29.3                                         20           9.8          9.1    50.7                                         40           40.1         61.9   10.2                                         80           40.1         22.7   3.6                                          200          8.5          3.9    .8                                           pan          .2           .2     .3                                           ______________________________________                                         Batch Conditions: 66% water, 2B pulley arrangement, 2-2.5 min.           

This relationship between the ammeter readings and the particle sizeindicates a definite shift to a small sized pellet distribution forlower ammeter readings. The amperage required for a particular pelletsize varies for different pulley or rotor configurations as furthershown in Table 2 above.

As discussed above, a property specifically required by the pelletizedsilica was that it be easily dispersible in rubber compounding mixtures.To test dispersability, an experimental matrix of samples was utilizedwhich consisted of four different densities, 0.2 g/ml, 0.37 g/ml, 0.25g/ml and 0.29 g/ml, in three different sizes, 6×10, 10×20, and 20×40.Nine of the twelve samples were subjected to a dispersion test into arubber with a Banbury Mill. These samples were quantitatively evaluatedand ranked from best dispersion to worst, i.e., from 1 through 11utilizing two ranking schemes A and B. Some of these samples were alsodispersed utilizing a two roll mill with the same rubber formulation asthat tested with the Banbury Mill The results of these tests are listedin Table 4 below.

                                      TABLE 4                                     __________________________________________________________________________    Dispersion, Size, and Density Data of Pelletized Silica                       DENSITIES         BANBURY MILL DISPERSION                                     (g/cc)       MESH RESULTS          TWO ROLL MILL DISPERSION                   16 × 18                                                                      POUR                                                                              PACK                                                                              SIZE RANK A   RANK B  RESULTS                                    __________________________________________________________________________    .25  .27 .29  6 × 10                                                                       9       3       4                                          .25  .29 .31 10 × 20                                                                      10       6       --                                         .25  .29 .32 20 × 40                                                                       9       1       --                                         .32  .37 .40  6 × 10                                                                       8       8       6                                          .32  .37 .40 10 × 20                                                                      10       7       6                                          .32  .36 .39 20 × 40                                                                      10       9       --                                         .23  .25 .28 10 × 20                                                                       9       4       4                                          .28  .29 .31  6 × 10                                                                      10       5       10                                         .28  .30 .33 10 × 20                                                                      10       10      6                                          .25  .29 .31 10 × 20                                                                      10       2       --                                         .32  .37 .40 10 × 20                                                                      10       11      --                                         .23  .25 .27  6 × 10                                                                      --       --      2                                          .23  .25 .29 20 × 40                                                                      --       --      3                                          .27  .29 .32 20 × 40                                                                      --       --      6                                          __________________________________________________________________________

In FIG. 3 the size of the pellets are graphed vs. their dispersionutilizing the rating results. As seen from FIG. 3 the graph showed nocorrelation between pellet size and dispersion, indicating that the ofsize of the pellets had no effect on the dispersion properties.

In FIG. 4 the density of the pellets are graphed vs. their dispersionratings. As illustrated in FIG. 4 the results show a correlation thatindicates a direct effect of density on dispersion such that the higherthe density, the poorer the dispersion. This correlation was notobserved in instances where a majority of the samples were rated aspoorly dispersible as in ranking scheme A. An unexpected result of thistest was that the two roll mill dispersed the pelletized silica betterthan the Banbury Mill.

One suspected variable affecting the pellet dispersion was the saltcontent of the silica. In order to investigate this possible effect ofsalt content on dispersion, pellets were made with various amounts ofNa₂ SO₄ and subjected to dispersion tests. The results of this study areillustrated in Table 5 below which shows that the results of thisinvestigation were inconclusive. While the lowest amount of sulfate(0.5%) did produce the lowest dispersion (4) this dispersion was farfrom what was anticipated. Furthermore, no direct correlation betweenthe amount of sulfate and the dispersion rating was observed. It wastherefore concluded that the amount of sulfate in a regular batch ofamorphous silica was not related to dispersability onto rubbercompositions.

                  TABLE 5                                                         ______________________________________                                        % Sulfate and Dispersion Ratings for Pelletized Silica                        % Sodium     Dispersion Rating                                                Sulfate      (Two Roll Mill)                                                  ______________________________________                                        0.5          4                                                                2.0          6                                                                3.8          5                                                                ______________________________________                                         Batch Conditions: 66% water, 2B pulley arrangement, 2.5 min., 2.4 amps.  

Besides dispersion, the quantity of dust associated with the product wasthe next important consideration regarding the pelletized silica. Dustis defined as silica pellets less than 200 mesh size. Previous work ondust tests defines it as the material capable of being suspended inwater with larger sizes settling out.

For purposes of this invention the following test was developed whichmeasured the suspension of dust in water with a HACH ratio turbidimeter.A standard graft was first developed in which a 1 gram sample ofground-up amorphous silica pellets were suspended in one liter of water.Four different aliquots of the sample were then diluted to 100millimeters. Each sample was placed in a HACH ratio turbidimeter andnephelometric turbidity units (NTU's) were read after five minutes. TheNTU readings were then plotted vs. percentage of dust to give a standardgraph.

The percent of dust was then calculated based on 5 grams of sample and60 grams of diluent water. According to the devised dust test procedure,five grams of pelletized silica were placed into an Erlenmeyer flaskcontaining 60 grams of water. The mixture was slowly stirred for 15seconds, then allowed to stand for an additional 5 seconds. Theresulting supernatant was then poured into a turbidimeter sample celland nephelometric turbidity units (NTU's) were read after five minutes.The percent of dust of the apelletized silica was read off of thestandard graph discussed above. The results are listed in Table 6 below.

                  TABLE 6                                                         ______________________________________                                        ml of 1% solution NTU's   % Dust                                              ______________________________________                                        17                9.8      .204                                               34                20      .41                                                 51                30      .61                                                 68                41      .82                                                 ______________________________________                                    

Once it was determined that the inclined mixer was capable of producingpelletized silica having suitable dispersion capabilities and acceptabledust levels, it was necessary to provide a suitable process for dryingthe pelletized silica.

As illustrated in FIG. 1 the pelletized silica products from mixer 2leave the mixer and enter into discharge hopper 7 which includes asuitable discharge means 8. The discharge means 8 directs undesiredproducts to a suitable collector 9 and transfers desired products to aconveyor means 10. Conveyor means 10 then transfers products to a dryer11 which, for purposes of the present invention, includes a fluidizedbed dryer.

It has been found for purposes of the present invention that themoisture content of the silica pellets formed in mixer 2 should have amoisture content of between about 60 and 68 percent water by weight. Thepellets are then dried in the fluidized bed dryer at a temperaturegreater than about 75° C. to a moisture content of about 5% by weight.In a preferred embodiment a vibrating fluidized bed dry was found to beparticularly useful for purposes of the present invention.

The present invention thus allows for the production of pelletizedsilica having a pour density between about 0.20 and about 0.50 g/cc, andmore preferably between about 0.25 and about 0.37 g/cc and an averageparticle size between about 30 and about 150 mesh, and more preferablybetween about 40 and about 80 mesh. Additionally, the pelletized silicacontains between about 0.1% and about 0.9% dust, and more preferablybetween about 0.2% and about 0.8% dust.

These properties of the pelletized silica product are controlledaccording to the present invention by utilizing an inclined mixer inwhich to pelletize the silica end by controlling the percent of solventadded to the silica charge in the mixer, the mixing speed of the stirrerand the batch time. In particular, the percent of solvent is betweenabout 40 and about 75% by weight and more preferably between about 56and about 69% by weight. The mixing speed of the stirrer is betweenabout 400 and about 3500 RPM, and more preferably between about 480 andabout 3225 RPM. Finally, the batch time in the mixer is 30 minutes orless.

As described in detail above the present invention is directed to aprocess for producing pelletized materials, particularly silica, whichare highly dispersible in rubber formulations. This pelletizing processinvolves producing the pelletized materials in an inclined mixer anddrying the pelletized products in a fluidized bed dryer. For purposes ofthe present invention an Eirich mixer was utilized as a preferred mixer.

As discussed above, the present invention is particularly concerned withproducing a highly dispersible pelletized product. In this regard it wasdiscovered that the dispersability of the pelletized products could becontrolled by controlling the density of the pelletized products formedin the inclined mixer. The density of the pelletized products wascontrolled by controlling the amount of solvent added to the mixerduring the pelletization operation and also by controlling the mixerspeed.

The pelletization process can be used for a variety of rubbercompounding materials including fillers, reinforcing agents, processingaids, pigments and mixtures thereof. However, the inventive process wasparticularly found to be useful for producing pelletized amorphoussilica.

In producing pelletized amorphous silica according to the presentinvention amorphous silica is combined in the incline mixer with betweenabout 55 and about 75% water as a solvent, by weight. Afterpelletization in the inclined mixer the pelletized silica is dried inthe fluidized bed dryer to a moisture content below about 10% by weight.Although it was discovered that the size of the pelletized product didnot affect the dispersability, the pelletized silica produced wascontrolled to have an average particle size between about 20 and about80 mesh. According to the present invention it was discovered that thedispersability of the pelletized silica was a function of the densitythereof. Accordingly, pelletized silica was controlled during theprocessing to have a density of between about 0.25 and about 0.40 g/ml.

In addition to the above process the present invention particularly isdirected to a novel form of pelletized amorphous silica which is highlydispersible in rubber formulations and is substantially dust free.

The following example is presented to illustrate the invention which isnot intended to be considered as being limited thereto. In the examplesand throughout percentages are by weight unless otherwise indicated.

EXAMPLE 1

In this example amorphous silica was mixed with 66% by weight of waterin an Eirich mixer model R-18 whose rotor rpm was 3,225 m/sec. Themixing operation produced pelletized silica having a particle size ofbetween 20-30 mesh (U.S. standard) at a 60% yield with the mixer's motoramperage reaching 2.4 amps. The pellets were dried in a fluidized beddryer operating at 80° C. at a rate of 22.9% weight loss per hour ofmoisture to give a highly dispersible product. It was found in thisexample that the dispersion was progressively hindered by increasing thedensity of the pellets, by slowing the rate of drying or by increasingthe drying temperature.

EXAMPLE 2

In this example the mixer of FIG. 1 was charged with 620 pounds of waterfrom the solvent source 4. The solvent was mixed with a 290 pound chargeof amorphous from the silica hopper. The mixer operated at 7 batches perhour or at about 8.57 minutes per batch. Properly sized pelletizedproduct having a 68% moisture content by weight was discharged onto theconveyor means and fed into the fluidized bed dryer at a rate of 6375pounds per hour and dried to form the final product.

From the above examples it is seen that the pelletized products of thepresent invention are characterized as having superior dispersionproperties together as well as a low percentage of dust and are easilyproduced by controlling process variables.

Although the invention has been described with reference to particularmeans, materials and embodiments, from the foregoing description oneskilled in the art can ascertain the essential characteristics of thepresent invention and various changes and modifications may be made toadapt the various uses and characteristics thereof without departingfrom the spirit and scope of the present invention as described in theclaims that follow.

I claim:
 1. A process for producing substantially dust-free pelletizedmaterials which are highly dispersible in rubber formulations whichcomprises pelletizing rubber additives in an inclined mixer and dryingthe pelletized product in a fluidized bed dryer.
 2. A process forproducing pelletized materials according to claim 1, wherein said mixeris an Eirich mixer.
 3. A process for producing pelletized materialsaccording to claim 1, wherein the dispersability of said pelletizedmixture is controlled by controlling the density of said pelletizedproducts.
 4. A process for producing pelletized materials according toclaim 3, wherein the density of said pelletized products is controlledby controlling an amount of solvent added to said mixer during saidpelletizing operation and by controlling the mixer speed.
 5. A processfor producing pelletized materials according to claim 1, wherein saidpelletized material is selected from the group consisting of fillers,reinforcing agents, processing aids, pigments and mixtures thereof.
 6. Aprocess for producing pelletized silica having a density between about0.20 and 0.50 g/cc and an average particle size between about 30 andabout 150 mesh which comprises pelletizing a mixture of amorphous silicaand solvent, having between about 40% and about 75% by weight ofsolvent, in an inclined mixer at a mixing speed of between about 400 andabout 3500 RPM and drying the pelletized silica in a fluidized bed dryerto a moisture content below about 10% by weight.
 7. A process forproducing pelletized silica according to claim 6, wherein saidpelletized silica contains between about 0.1 and 0.9% dust.
 8. A processfor producing pelletized silica according to claim 7, wherein saidpelletized silica contains between about 0.2% and about 0.8% dust.
 9. Aprocess for producing pelletized amorphous silica according to claim 6,wherein said mixer is an Eirich mixer.
 10. A process for producingpelletized amorphous silica according to claim 6, wherein said driedpelletized silica has a moisture content of below 5 percent by weight.11. A process for producing pelletized amorphous silica according toclaim 6, wherein said pelletized silica has an average particle sizebetween about 20 and about 80 mesh.